Method of crosslinking polyolefins

ABSTRACT

A method of forming an engineering component comprising the steps of:  
     subjecting a workpiece or blank formed from polyolefin to gamma radiation at a total dose sufficient to cause a predetermined degree of crosslinking, wherein the total dose is applied at a dosage rate of less than 5 kGy/hour to cause crosslinking of the polymer and forming an engineering component from the crosslinked material.

[0001] This invention relates to the method of crosslinking polyolefins, particularly but not exclusively polyethylene using gamma radiation. Cross-linking is beneficial as it improves the wear resistance of polyethylene used in orthopaedic implants and other engineering applications, especially ultra-high molecular weight polyethylene (UHMWPE) used for these applications.

[0002] The crosslink density, that is the distance between bonds, is proportional to the radiation dose received. Wear resistance increases with higher crosslink density that occurs following use of higher doses of radiation. The detrimental counter effect of crosslinking is to reduce many mechanical and physical properties of the polymer. This reduction also occurs in proportion to the dose received. Higher doses cause greater reduction in physical properties. A reduction in strength may lead to a physical failure of the component. The maximum dose, and hence the maximum enhancement of wear that can be used in a particular circumstance is limited by this reduction in other physical properties. Commercial highly crosslinked UHMWPE has previously been treated with a specified total gamma radiation dose. The actual dose rate has not been considered important and has generally been left to the convenience of a vendor or contractor. Typically the crosslinking of polyethylene has been carried out at a dose rate of about 5 kilo Gray (kGy) per hour or higher (0.5 MRad/h).

[0003] Accordingly to the present invention a method of forming an engineering component comprises the steps of:

[0004] subjecting a workpiece or blank formed from polyolefin to gamma radiation at a total dose sufficient to cause a predetermined degree of crosslinking, wherein the total dose is applied at a dosage rate of less than 5 kGy/hour to cause crosslinking of the polymer and forming an engineering component from the crosslinked material.

[0005] In a preferred method the dosage rate is less than 3 kGy/hour, more preferably less than 1.5 kGy/hour.

[0006] The dosage time is adjusted to provide a sufficient total dosage to cause efficient crosslinking and sterilisation. The total dosage may be selected by conventional means. A common total dose of 100 kGy may be used, although doses from 40 kGy to more than 102 kGy may be used for UHMWPE for orthopaedic prostheses and implants.

[0007] By reducing the dose rate to below 5 kGy/hour, preferably below 3 kGy/hour and most preferably below 1.5 kGy/hour, there is a significant improvement in the mechanical properties, particularly the crystallinity, impact strength and elongation at break. Conventional cross-linking by irradiation at higher dosage rates may decrease the crystallinity of UHMWPE from 50% to 35% for a total dose of 100 kGy. The use of the lower dose rate in accordance with this invention can maintain a level of crystallinity over 40%, leading to a consequent reduction in the loss of impact strength and elongation at break; The loss of these mechanical properties has been found to be less at lower dose rates for the same total dose level.

[0008] The polyolefin is preferably a polyalphaolefin, preferably selected from polyethylene, polypropylene and copolymers and blends thereof. Use of ultra high molecular weight polyethylene with molecular weight>1×10⁶ g/mol preferably>3×10⁶ g/mol is especially preferred.

[0009] The invention is further described by means of example but not in any limitative sense.

EXAMPLE 1

[0010] Ultrahigh molecular weight polyethylene (UHMWPE) was crosslinked by gamma radiation from a cobalt 60 source at four different dose rates to the same total dose (100 kGy). The dose was assessed by dosimeters in accordance with BS EN 552. The exercise was repeated to provide 3 sets of test materials.

[0011] The degree of crosslinking (cross-link density) was measured using a SRT 1 (Swell Ratio Tester) supplied by Cambridge Polymer Group of Sommerville Mass., USA to the draft ASTM standard D2765¹ in accordance with the procedure used for the round robin tests. The four samples within each set were the same (no significant difference) thus demonstrating the same crosslink density and no effect of dose rate.

[0012] Mechanical and physical properties were measured in accordance with the standards stated in Table 1 and the results analysed using Student's t test for matched pairs to demonstrate significance.

[0013] The dose rate was demonstrated to have a significant¹ effect on the Impact Strength, Elongation at break and Crystallinity. Lower dose rates provided materials with significantly better properties than those produced at high dose rates. TABLE 1 Cystal- Impact Elongation Dose Rate Swell Crosslink linity Strength % kGy/hr Ratio Density % kJ/m² ASTM F Method Calculation ASTM D2765² D.S.C³ Izod⁴ 648 DR1 1.0 3.1 0.15 42.3 63 239 DR2 1.8 3.0 0.16 38.3 62 235 DR3 6.1 30 0.165 36.7 58 234 DR4 7.3 3.1 0.155 34.4 59 224

EXAMPLE 2

[0014] Orthopaedic grade, ram extruded GUR 1050 rods of 65 mm diameter were manufactured for gamma irradiation in air to a dose level of 100 kGy. Mapping of the irradiation plant was carried out using dosimetry to determine the placement of rods to achieve the specified nominal dose rates. Each set of four rods were irradiated at different nominal dose rates of 1, 2, 6, and 7.5 kGy per hour. TABLE 2 Actual Dose Level Nominal Dose Rate KGy Range Dose rate kGy/hr kGy/hr EN 552 kGy 1.0 1.0 99.9-101.4 1.5 2.0 1.8 97.7-100.8 3.1 6.0 6.1 95.8-102.7 6.9 7.5 7.3 96.3-104.8 8.5

[0015] The rods were melt annealed in an air atmosphere at 150 C. with a slow cool down rate to ambient temperature. The rods were machined into test specimens with a minimum sample size of six for each dose rate. Tensile Strength, Yield Strength and Elongation at Break were determined in accordance with ISO 527 using Type 5 specimens. Impact Strength testing conformed to ASTM F648-00 Annex A1 using double notch Izod specimens. Crosslink density and swell ratio was determined using the SRT-1 (Cambridge Polymer Group) to the draft ASTM standard 3.2 Mar. 1, 2001. Samples of 150 μm were tested on a Nicolet FTIR with microscope to determine the Transvinyl Index (TVI) (Muratoglu, O.K.et al., 47^(th) ORS 2001 p. 1013) and a Netsch Differential Scanning Calorimeter to determine the crystallinity. Gamma irradiation using the above mentioned dose rates and testing was carried out on three independently crosslinked sample sets. Statistical analysis was performed using Graphpad software and a p-value <0.05 was used to establish significance. A comparison was made to rods irradiated during a production run of two hundred rods of the same diameter and dose level, but using a dose rate of 0.4 kGy per hour. TABLE 3 Radiation Rate in kGy/Hour Property 0.4 1.0 1.8 6.1 1.8 Impact 64 63 62 59 59 Strength kJ/m² Yield 20.4 19.6 19.5 19.4 19.3 Strengh MPa Tensile 44.2 43.2 41.3 41.1 42.8 Strength MPa Elongation at 246 239 236 234 224 break % Swell Ratio 3.04 3.14 3.00 2.97 3.06 Cross-link 0.16 0.15 0.16 0.16 0.16 Density Mole/dm³ Crystallinity 43.0 42.3 38.3 36.7 33.9 %

[0016] Izod impact strength, elongation at break, yield strength and crystallinity showed a significant decrease with increasing dose rate (p<0.05). The square of the correlation coefficient (R²) for yield strength versus crystallinity was 0.9985. The square of the correlation coefficient (R²) for Izod impact strength versus dose rate was 0.9998. The level of crystallinity reduced by 20% with increasing dose rate whilst swell ratio and cross-link density showed no statistical significance between dose rate. 

1. A method of forming an engineering component comprising the steps of: subjecting a workpiece or blank formed from polyolefin to gamma radiation at a total dose sufficient to cause a predetermined degree of crosslinking, wherein the total dose is applied at a dosage rate of less than 5 kGy/hour to cause crosslinking of the polymer and forming an engineering component from the crosslinked material.
 2. A method as claimed in claim 1 wherein the dosage rate is less than 3 kGy/hour.
 3. A method as claimed in claim 2 wherein the dosage rate is less than 1.5 kGy/hour.
 4. A method as claimed in a preceding claim wherein the polyolefin is an unblended homopolymer.
 5. A method as claimed in claim 4 wherein the polyolefin is polyethylene.
 6. A method as claimed in claim 5 wherein the polymer is ultra high molecular weight polyethylene.
 7. A method as claimed in any preceding claim wherein the crystallinity of the crosslinked polyolefin is at least 40%.
 8. An engineering component comprising polyolefin crosslinked in accordance with the method of any preceding claim.
 9. A surgical implant or prostheses comprising a polyolefin crosslinked in accordance with the method of any of claims 1-7. 